Electroluminescent-photoconductive device with improved linearity response



E C I m Dm? W05 IP9 Tl NMR ODYS SNT O l NCT..1 lomp LMEA MHN OPM@ TL. 1NDi BEEF .mmm TMPn IM MIg U i LHr OTO RI Tw C E L E June l, 1965 FIG. 2

FIG. l

vw L luminescent apparatus constructed in accordance withY the presentinvention, and i FIG. 4 illustrates a modified form of the apparatus ofFIG. 3. Elemental apparatus f FIG. I

Referring to FIG. 1 of the drawing, there is shown an elemental form ofelecrtoluminescent apparatus constructed in accordance with the presentinvention. This apparatus of FIG. 1 is referred to as being elemental inYnature because, for the .case of a complete image, it is capable ofreproducing only a single image element, This is not, however, -intendedto imply anyrlimitation as to the physical size of the apparatus.V Noris it in 7 tended to imply that the apparatus is only useful forreproducing intelligible images because, for example, the apparatusmight bek used as a radiationidetector or amplier where no image isinvolved. Also, for the sake of a concrete illustration of its use, theapparatus of FIG. 1, as well as that of the other figures, shall ingeneral be described for Vthe case where the radiation of interest isvisible light radiation.Y It is to be understood that the apparatus isequally useful with other types of electromagnetic radiation Vsuch asX-rays, ultraviolet radiation, or infrared radiation. g

Considering now the details of the electroluminescent apparatus of FIG.1, such apparatus comprises an electroluminescent cell 10 of the formcomprising a-quantity of electroluminescent material 11 ypositionedbetween-a pair of conductive electrodes 12 and 13. Theelectroluminescent material 11 may be, for example, copperactivated zincs ulde. Also, one of the conductive electrodes, in this case theelectrode 12, is preferably made to be radiation transparentfor enablingthe radiant glow of the electroluminescent material 11 to escape fromthe cell 19. As is known in the art, such radiation transparentconductive electrode may consist of a very thin metallic lm of, forexample, aluminum or a film of transparent conductive material such astin oxide. Instead, such electrode might take the form of a grid meshstructure wherein the sizes of thevapertures are arranged so that theelectrode is essentially transparent. In orderv to indicate that suchelectrode is radiation transparent, it is represented diagrammaticallyby means of a dashed line. Such conductive electrode may be formed Yon asheet of dielectricrnaterial 14 which,for the case of visible light',may take the form of glass or a thin layer of mica. In the case of theFIG. 1 apparatus, the other conductive electrode 13 may be composed of`a Y relatively thick layer of metal and need notbe radiation to varythe impedance of the photoconductive material 16. This electrode 18 maytake any of the forms previously mentioned in connection with thetransparent electrode 12. of the cell 1G. Also, the other electrode 17of cell 15 mayagain be a relatively thick layer of metal.

The apparatus also includes an electrical impedance represented, forexample, by a resistor 2) and means for supplying an energizing voltage.The energizing voltage should, at least on the basisrof presentknowledge, be of Ya uctuating nature and, hence, `may take the form ofan alternating-current voltage of suitable frequency. Means forsupplying such an energizing voltage is represented diagrarnmatically byan alternating-current voltage generator21 having a'pairof outputterminals 22 and 23.

The apparatus of the present invention further includes means connectingthe two cells in parallel with one another and in series with thevimpedance and the voltage-supply means 21.11 Such connecting meansincludes a connecting wire 24 for connecting the two electrodes 13 and17 to the terminal 23 of the voltagesupply means and includes connectingwires 25, l26, and 27 for connecting the other two electrodes, namelythe conductive electrodes 12 and 18, to the other terminal 22 of thevoltagesupply means. It is essential that the impedance represented inthis case by the resistor `20 be connected in series so that the currentflow to both the photoconductive material 16 and the electroluminescentmaterial 11 must pass therethrough. In the case shown, this isaccomplished by connecting the two wires and 26 to the side of theresistor 20 farthest removed from the voltage source 21. Y

Considering now the operation -of the electroluminescent apparatus justdescribed, it will be noticed that the electroluminescent cell. 10 andthe photoconductive cell 15 are connected in parallel with one `anotherand that this parallel combination is, in turn, connected to the voltagesource 21 by way of the series impedance represented by resistor 20. Asa result, the apparatus operates in the bright condition, that is tosay, the condition in which it is highly sensitive to voltage changes,when the intensity of incident radiation I is small and the.photoconductive element 16 is most sensitive to changes inthe incidentradiation I. By employing an applied voltage and seriesconnectedimpedance of suitable values, there can be made to occur a more linearchange in the light output L in response to changes in the incidentradiationI than can be obtained inrmost known devices of 4the typeheretofore proposed where the photoconductive element and theelectroluminescent element are connected in series with each other.

In operation, with no incident light I falling on the photoconductivelayer 16 through the transparent electrode 18, there is a high impedancebetween the electrodes 17 and 18 so that only a small current lilows inthe circuit and a relatively high voltage is applied across theelectroluminescent layerll located between the electrodes 12 and 13. Thevalue of the voltage actually appearing across the electroluminescentlayer 11,-of course, depends on the relative impedances of the seriesresistor 20 and the electroluminescent layer 11 Vat the supplyfrequency. To this end, the construction of the layers and the impedanceandsupply voltage values are chosen so that the electroluminescentlayer11 is operating at the maximum brightness required for this condition ofno incidentA illumination.

Then, as gradually increasing amounts of incident light, as representedby the arrows I, are allowed to fall on the photoconductive layer 16,`the resistance of the photoconductive materialV decreases. 'The currentthrough the resistor 20 thereupon increases and the voltage across theelectroluminescent layer `11 is'reduce'd,l thereby resulting in adecreased light output L therefrom.

Although the changes in the lightY output L will be in the reversedirection to changes in the incident radiation I, Iby employing twounits of apparatus, each constructed in accordance vwith FIG. 1 andarranged so that the light L emitted by the electroluminescent elementof the first is directed onto the photocoriductive element of thesecond, an overfall operation is obtained inwhich the light outputvaries in :the same sense as theincident radiation.

energies Witha single unit of apparatus, as shown 1n FIG. 1, a moreuniform change in light output L with steady changes in the incidentlight I on the photoconductive layer 16 is obtained. This can be morereadily appreciated by reference to the graph of FIG. 2 which shows acurve relating the brightness of the light output L fromelectroluminescent cell l@ to the RMS. value of the alternating-currentvoltage applied across such an electroluminescent cell 10 at a constantfrequency. It caribe seen that as the voltage is increased from zerothere is an apparent threshold Voltage V1 at which a significant amountof light is lirst emitted by the cell 10. The rate of change ofbrightness with respect to voltage, that is, the incremental slope ofthe curve of FlG. 2, then increases as the voltage is increased butsubsequently decreases after a voltage V2 is reached. This continuesuntil the cell 10 is operating near the maximum brightness obtainable atthe frequency employed.

For the case of prior art devices of the type previously proposed wherethe photoconductive layer and the electroluminescent layer are connectedin series, the relative impedances are such that without employing asupply voltage of an undesirably high value onli,7 a relatively smallvoltage is applied across the electroluminescent layer when the incidentillumination on the photoconductive layer is small. This situationcontinues until the voltage across the electroluminescent layer becomesgreater than the effective threshold voltage V1 and, until thisthreshold voltage is exceeded, no significant amount of light will beemitted by the electroluminescent layer. Furthermore, in such prior artdevices further increases in the amount of incident lightresult inhigher voltages being applied across the electroluminescent layer but itcan be seen that changes in the amount of light emitted for uniformchanges in the incident light become gradually greater and asubstantially linear variation in the light output with uniform changesin the amount of incident light is not obtained.

With the shunt-connected or parallel-connected apparatus of the presentinvention, on the other hand, the applied Voltage is chosen so that theelectroluminescent cell 10 operates in the region of V2 when there islittle or no incident illumination on the photoconductive cell. Anincrease in the incident illumination then causes the operating Voltageacross the electroluminescent cell l0 to decrease towards V1. As aresult of this mode of operation, the threshold effect is substantiallyeliminated for incident radiation of weak intensity. Also, both theelectroluminescent cell lil and the photoconductive cell 15 are in theirmost sensitive condition when the incident radiation is small.Furthermore, by using elements of suitable materials and dimensionstogether with a series impedance of an appropriate value, changes in theincident radiation produce a substantially linear change in thebrightness of the electroluminescent cell 1G over anY appreciable rangeof values. Also, as mentioned, by employing two units of apparatus withthe light emitted by the electroluminescent cell of the first unitarranged to be incident on the photoconductive cell of the second unit,an output which is substantially directly proportional to the incidentlight can be obtained. By a suitable choice of circuit components, alight-amplification edect may also be obtained.

Another feature of the present invention is that it makes practical theuse of radiation feedback from the electroluminescent cell to thephotoconductive cell of the same -unit of apparatus without impairingthe operation ofthe apparatus. As a result, an even more linear changein the light output in response to changes in the incident illuminationmay be obtained. This, of course, is only applicable where thephotoconductive material is sensitive to the same type of radiation asemitted .by the electroluminescent material. The amount of feedback may,of course, be controlled by limiting the amount of light from theelectroluminescent element whichV is allowed to fall on thephotoconductive element. The light feedback will, of course, vary in thereverse direction tovariations in the incident radiation, that is tosay, it will constitute negative feedback and, by controlling the amountof feed-V back, for example, by the use of a screen of a suitabletransparency, a variable light output giving a substantial linearresponse to changes in the input radiation can be obtained. This form ofoperation may most readily be obtained by using a modified form ofconstruction as will now be explained in connection with EEG. 3.

Elemental apparatus of FIG. 3

Referring now to FG. 3 of the drawing, there is shown a further form ofelemental.electroluminescent apparatus constructed in accordance withthe present invention wherein the electroluminescent and photoconductive cells are, so to speak, arranged back-to-back to one anotherwith a single conductive electrode common to each cell. Theelectroluminescent cell includes a quantity of electroluminescentmaterial 39 positioned between a pair of conductive electrodes 31 and32. Similarly, the photoconductive cell includes a quantity ofphotoconductive material 33 positioned between a pair of conductiveelectrodes 32 and 34. As is apparent, the center conductive electrode 32is thus an electrode which is common to the two cells. The outer twoconductive electrodes 3l and 34 are radiation transparent and may besuperimposed on corresponding layers 35 and 36 of a dielectric materialsuch as glass. The series impedance is, in this case, repf resented by acondenser 37" and is connected in series with the voltage-supply means21 and its terminals 22 and 23.

The operation of the apparatus of FlG. 3 is generally the same as thatof FIG. 1 apparatus because of the fact that the electroluminescent andphotoconductive cells of FIG. 3 are effectively coupled in parallel withone another. There is one important exception, however, in thatradiation f edback from the electroluminescent layer 30 to thephotoconductive layer 3.3 automatically occurs unless some means isincluded for blocking such feedback. As mentioned, such feedback isdesirable in that it serves to improve the linearity of the device.Where no such radiation feedback is desired, however, such feedback maybe prevented by including a thin layer of opaque material between eitherthe electroluminescent layer 3i? and the conductive electrode 32 orbetween the conductive electrode 32 and the photoconductive layer 33 orboth. Another way of achieving the same result would be to make thecommon conductive electrode 32 of suti'icient thicknessso as to besubstantially opaque to the radiation emitted by the electroluniinescentlayer 3?. 0f course, in most cases the radiation feedback,

which is a form of negative'feedback, will be desired.

The amount of such feedback may be controlled by either suitablyselecting the transparency factor of the common `conductive electrode 32or by utilizing semitransparent Elemental apparatus of FIG.

Referring now to FIG. 4 of the drawing, there is shown a modified formof electroluminescent apparatus which is generally similar to that ofFIG. 3 except that Vprovision has been made whereby the desired seriesimpedance is built into the device itself thus eliminating the need foran external circuit component. To this end,

each of the dielectric layers 35 and S may include additional conductiveelectrodes 4t? and il positioned adjacent the outer surfaces thereof.These additional elecenergie/i trodes 40 and 41, which shouldberadiation transparent, may be formed, for example, by coating suitablefilms of transparent conductive material on the outer surfaces ofthedielectric layers V35 and 36. These additional conductive electrodes 40and 41 are Vthen coupled Vto the terminal 23 of the voltage-supplymeans. In addition, what was formerly the outer conductiveV electrodes,namely the electrodes 31 and 34, are connected together, for example, bythe external connecting wire 42.

In operation, the capacitance across each oi` the dielectric layers 35and 36 serves to form the desired series impedance which is connected inseries between the voltage-supply means 21 and the parallel-connected'electroluminescent and photoconductive cells. It is essential that theconductive electrodes 31 and 34 be connected together as indicated bythe external connecting wire 42 in order that this built-in capacitanceof the dielectric layers 35 and 36 `may be common to both theelectroluminescent and photoconductive cells. In other words, thecapacitances of the dielectric layers 35 and 36 are effectively inparallel with one another and this parallel combination is in serieswith the parallel combination formed by the electroluminescentiandphotoconductive cells. if desired, either the conductive electrode 40 orthe conductive electrode 41 may be omitted, in which case the seriescapacitance is correspondingly reduced.

Y It will be appreciated, of course, that electroluminescent apparatusconstructed in accordance with the present invention are not restrictedto casesin which the incident radiation lies within the visible rangebut may also be used to convert nonvisible radiation such as, forexample, X-rays, ultraviolet radiation, yor infrared radiation intovisible radiation `by use of an appropriate photoconductive material. Y

While there have been described what are at present considered to be thepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and rnodiiicationsmay be madetherein without departing from the invention, and it is, therefore,aimed yto cover all such changes and modifications as tall within theYtrue spirit and scope of the invention.

What is claimedisi' 21. `An electroluminescent device comprising contiguousV layers of different materials positioned one after the other inthe lfollowing order: a first layer of conductive material; a layer ofphoto-impedance material; a second layer of conductive material; a layerof electroluminescent material; a third layer of conductive material;first andv sec-V ond layers of dielectric material contiguous with theouter side of said rst and third conductive layers, respectively; andfourth and iitth layers of conductive material and means connecting thesecond conductive `layer to another terminal ofthe voltage-supply means.

3. Electroluminescent apparatus comprising; an electroluminescent cellcomprising a quantity of electroluminescent material positioned betweena pair of conductive electrodes, a iirst of these electrodes beingradiation transparent for enabling the radiant glow of theelectroluminescent material to escape from the `cellga photo-irnpedancecellicomprising a quantity of photo-impedance material positionedbetween a pair of conductive electrodes, a iirst of these electrodesbeing radiationk transparent for enablingy incident radiationzto varythe impedance of the photo-impedance material; the cells being locatedadjacent one another so that a single common conductive electrode formsthe second electrode for both cells, the radia-tion transparency yfactorof this common electrode being chosento tix the radiation feedback fromthe electroluminescent material to the photoimpedance material at aproper value for improving the response linearity of the apparatus;'anelectrical impedance; means for supplying an energizing voltage; andmeans connecting the two cells in parallel with one another and inrseries with the impedance and Ithe voltage-supply means,

contiguous withvthe other side of said rst .and second Y layers ofdielectric material, respectively.

2. Electrolulminescent apparatus comprising: contiguous layers ofdifferent materials positioned' one after the second layersof'dielectric material, respectively; means for supplying an energizingvoltage; means/,connecting the iirst and third conductive layers -to oneanother'so as to be electrically common; means connecting the fourthconductive layer to one terminal of the voltage-supply means;

whereby variations in the intensity of the radiation incident on thephoto-impedance material will cause the intensity of glow ofthe'electroluminescent material to vary in an inverse manner.

4. Electroluminescent apparatus comprising: an electrolurninescent cellcomprising a quantity of electroluminescent material positioned ybetweena pair of conductive electrodes, a first of these electrodes beingradiation trans-Y parent' for enabling the radiant glow of theelectroluminescentl material to escape from the cell; a photo-irnpedancecell comprising a quantity of photoimpedance material positioned.between a pair of conductive electrodes, a liirst of these electrodesbeing radiation transparent for enabling incident radiation to vary theimpedance of the photo-impedance material; the cells being locatedadjacent one another kso that a single common radiation transparentconductive electrode forms the second electrode for both cells; aquantity of semitransparent material located between at least one ofsaid electroluminescent and photo-impedance materials and said commonelectrode for fixing the radiationfeedback from the electroluminescentmaterial to the photo-impedance material at .a desired value [forimproving the response linearity of the apparatus; an electricalimpedance; means for supplying .an energizing voltage; and meansconnecting the two cells in parallel with one another and in series withthe impedance and the voltage-'supply means, whereby variations in theintensity of the radiation incident on the photo-impedance material willcause the intensity of glow of the electroluminescent material to varyin an inverse manner.

References Citedbythe Examiner UNITED STATES PATENTS 2,882,419 4/59Diemer et al Z50-213 2,886,556 4/59 Jenny et al. Z50-213 2,896,087 7/59Kazan Z50-213 V2,896,088 7/59 Lempert 250-213 V2,906,884- 9/59 Gill250-213 2,909,667 10/59 Orthuber et al. Z50-213 2,914,679 1l/59 Loebner250- 213 2,987,624 6/61 Diemer 250--213 RALPH G. NusoN, Primm Examiner.ARCI-IIE R, BORCHELT, Examiner,

1. AN ELECTROLUMINESCENT DEVICE COMPRISING CONTIGUOUS LAYERS OFDIFFERENT MATERIALS POSITIONED ONE AFTER THE OTHER IN THE FOLLOWINGORDER; A FIRST LAYER OF CONDUCTIVE MATERIAL; A LAYER OF PHOTO-IMPEDANCEMATERIAL; A SECOND LAYER OF CONDUCTIVE MATERIAL; A LAYER OFELECTROLUMINESCENT MATERIAL; A THIRD LAYER OF CONDUCTIVE MATERIAL; FIRSTAND SECOND LAYERS OF DIELECTRIC MATERIAL CONTIGUOUS WITH THE OUTER SIDEOF SAID FIRST AND THIRD CONDUCTIVE LAYERS, RESPECTIVELY; AND FOURTH ANDFIFTH LAYERS OF CONDUCTIVE MATERIAL CONTIGUOUS WITH THE OTHER SIDE OFSAID FIRST AND SECOND LAYERS OF DIELECTRIC MATERIAL, RESPECTIVELY.